X-ray tube



w m mm J. H. WAN EH TUUIK- mm,

' X-RAY TUBE Film April 18, 19

Patented Jan. 31, 1950 UNITED STATES PATENT OFFICE X-RAY TUBE trustee Application April 18, 1946, Serial No. 662,962 In the Netherlands November 20, 1941 Section 1. Public Law .690, August 8, 1946 Patent expires November 20, 1961 6 Claims.

In order to prevent the wall of X-ray tubes from being struck by electrons a hood is often placed on the anode, which hood exhibits opposite the cathode an aperture for the cathode rays and a lateral window through which the useful X-rays can emerge. By this cap secondary electrons are collected for the greater part thus preventing dangerous loads on the wall of the tube.

When securing such a hood to a rotary anode it must have an aperture for the cathode rays whose diameter corresponds at least to that of the path covered by the outermost point of the focal spot. In this case the hood is too wide for collecting secondary electrons to a suflicient degree.

The invention concerns an X-ray tube having a rotary anode furnished with a screen which surrounds the front surface of the anode as a hood and exhibits opposite the cathode an aperture for the beam of primary electrons. However, this screen does not follow the rotation of the anode so that the aperture does not transpose relatively to the cathode and need not exceed the section of the electron beam having to pass it. It extends completely beyond the axis of rotation and in this way the screen collects a much larger part of the electrons emitted by the heated part of the anode than if it would follow the rotation of the anode and would require a wide concentric aperture. In addition, it collects metal particles disintegrating from the anode surface and breaks undesirable discharges.

It has occasionally been proposed to cover the front surface of a rotatably mounted anode by a screen (see French patent specification 713,190) which deals with a tube for scientific research by means of characteristic rays. To prevent as much as possible the formation of a deposit on the front surface of the anode it was covered, except for the focal spot, by a stationary screen. If this screen had to suit as much as possible the purpose for which it was used it had to approach as closely as possible to the anode surface to be protected. Consequently the aperture for the cathode-rays served at the same time for the emission of X-rays and electrons from the focal spot were not collected by thisscreen.

In contradistinction thereto the screen in the tube according to the invention must be placed at some distance from the front surface of the anode in order that electrons emitted from the focal spot on the anode may be checked as much as possible. Due to this the aperture for the beam of primary electrons cannot serve at the same time for the passage of the X-rays,

In regard to a screen following the rotation of the anode a stationary screen according to the invention still has other advantages than better checking the electrons and other particles from the focal spot. The surface of the anode heated by electron-impact conserves a high temperature for some time so that, after it has left the beam of cathode rays, it is able to emit electrons.

Consequently, if an electric field is produced at this surface, which is directed to the surface, such as is the case withalternating voltage during each wave half when the anode is negative relatively to the cathode, electrons are able to emanate from the whole annular part covered by the primary electrons. This is why in tubes having a rotary anode with a large heated surface it is desirable to make use of a rectifier. In addition the more or less rough surface of the anode mirror gives rise to the emission of electrons under the action of a field with a negative anode. A screen according to the invention weakens the field at the surface of the rotary- .anode mirror and even removes it over a large part. Consequently it reduces the risk of a strong current flowing in a wrong direction and permits the tube to be fed with alternating voltage and to omit the rectifier. The screen remains: at a comparatively low temperature so that it does not emit a considerable quantity of electrons. Its surface, where the lines of force end, is not attacked but remains smooth so that there is no risk of so-called cold emission.

The screen according to the invention acquires particular importance if it is extended to the rear and secured at the extremity, in contradistinction to the tube according to French pat. spec. 713,190 where it is attached at the middle, and more particularly with a good terminal contact, to the body from which is suspended the rotary part of the anode. In this case the rotary part is completely enclosed by the screen and then the latter is capable of absorbing the heat radiated by the rotor to the outside and of carrying it away to the supporting member, whence it can be transmitted to a cooler, such as the well-known highly upstanding ribs which fit with a small intermediate space in corresponding grooves of the rotor and serve to collect the radiated heat and to carry it off to the cooler. In order to promote this effect the hood is provided at the inner side and the rotor at the outside with a surface which is appropriate for absorbing and radiating heat respectively. To this end these surfaces, where no lines of force end, are roughened, for instance,

frosted or furnished with helical grooves and may, if desired,.be-coated'rwith a black layer. In order to preventitoo' much heat from being radiated by the screen to the glass wall of the tube the outer surface of the screen may be made highly res flective. This is also advantageous from an electrical point of view, since there are no strong concentrations of the electric held at a smooth surface. Consequently secondary and coldemis sion is counteracted by making the surface smooth.

One form of construction of an X-ray tube according to the invention is represented? in the accompanying drawing. In this drawing Fig. 1. is a longitudinal section of an X-ray tube, Fig. 2 being a cross-section of the same tube on the line II-II.

The wall of the tube consists of a metal middle :portion l and two glass: portions 2 and 3 sealed to. the. edges. thereof. The. anode consists of. a rotarypartlha supporting body E'and a screen 6. Thelatter. surrounds. the front surface of the ro tary Dartas av cap andexhibits opposite the cathode an. apcrtureB which lies entirel beyond the axis. of. rotation and permits the passage of the beam of' primary electrons. The part of the screen where is provided? the aperturea is located at somedistance from the. conical front surface of. the rotor. Laterally of this front surface the.

screen 6' exhibits a window 9. and opposite this window the metal part i has also a Window ill. These windows permit the passage of'the useful X-rays emitted by the tungsten anode mirror 5 E. To the supporting body 5' are secured a sealing edge l2; which similarly to the part I may consist of chrome iron, and a cooling rod iii. The glass part 2 exhibits a reentrant sealing tube M which is sealed to the rim l2.

The supporting body-5 contains two ball blocks [5 and IS in which is journall'ed a spindle H to which the rotor is'fixed-by means of a thin-walled metal tube l8.

The glass part 2' is surrounded by an electromagnetic system 191 by means of which a magnetic rotary fieldcan be produced by which the rotor iis actuated.

During operation of the tube the metal part 1 maybe connected to a point of the electric feed system which has about equal potential differences in regard to the anode and thecathode and is usuall'y connected to earth. The secondary and thermionically emitted electrons emitted by the strongly heated front surface of the rotor are collected for the greater part by the screen 6 and cannot involve an unduly strong current to the metal-part l. Inaddition they cannot form dangerous loads on the glass parts of the wall and on the metal part i if the latter is kept insulated. Owingto the screen a current between theanode and cathode in a wrong direction is not tube feared. Thespace between the screen and the anodemirror is field-free for the greater part so-thatthe tendency of thetube. to emit electrons fromthe anode is low.

Thev screen. t extends to the, rear and is connected witha good terminal'contact to the outer wall of the supporting body 5. The inner surfaceof thescreen opposite therotor and the outer surface-of the rotor-"5 opposite the screen are-s0 made asto readily absorb and radiate heat re spectively. They are preferably coated with a dull black layer. By this expedient it is achieved that the screen 6 readily takes up. heat from the rotor without contacting therewith. This heat is. carried. away to the supporting member b the screen 6 which preferably consists of copper. The part ofthesupporting bodyprojectinginto the rotor absorbs the heat radiated by the. inner surface of the rotor. In the same way, therefore, the opposed surfaces of rotor and supporting body can also be made appropriate for the radiation and absorbtion of heat.

The supporting body 5 has a ring 20 of ferromagnetic material through which the magnetic rotary field isqlcompleted. The lines of force of this field have to'penetrate through the stationary screen 6 and'may cause considerable Foucault currents therein which involves a decrease of the driving couple. In order to counteract this decrease the screen opposite to the poles of the magnet' system l9 (this will usually have not more than three poles in order to attain as high a sp e of. rotation as possible) ma be furnished with apertures for the passage of the magnetic flux. These apertures are designated by 2| in Fig. 2'.

V7 hat we claim is:

1. An X-ray tube comprising a-cathod'eelec trode, a rotatable anode electrode; and a shield member comprising a cylindrical portion surrounding the anode and a disc-shaped portion closing the cylindrical portion interposed between the anode and cathode electrodes; said. cylindrical portion being fixedly'mounted" to a support member which rotatably supports the anode whereb the rotatableanode is completely onclosed. by the shield, said disc shaipe-d portion having :an aperture therein to permit electronsemitt'ed by said' cathode elect'rodeto impinge on said anode electrode and said cylindrical portion having an'aperture therein to permit. the passage of X-rays from saidanoda.

2. An XJ-ray tube comprising an evaouatedtubular envelope havin a metallic wall. portion, a cathode electrode within said: envelope, a. rotatable anode electrode within said envelope,v and a shield member comprising a cylindrical portion surrounding the anodeandia: disc shaped portion closing said cylindrical:portioninterposedbetween the cathode and anode electrodes; said cylindrical portion beingfixedly-mountedt to a suppor member which rotatably supports the anode whereby the rotatable anode.- is completely enclosed-by the shield; said-.disc-shapedportion having' an aperture thereinv to permit electrons emitted by said cathode electrode-to impinge on said anode electrode: and said cylindrical portion having an aperture therein. aligned with an aperture in said metal Walt-portion to-permit thepassage of X-rays.

3. An X-ra tube comprising-an evacuated tubular envelope having a metallicv wall portion, a cathode electrode within saidenvelope, at rotatable anode electrode within saidenvelope; and a shield member comprising a cylindrical portion surrounding theanode and: adisc-shaped portion closing the cylindrical portioninterposed'between the cathode and'anode electrodes, said cylindrical portion. being fixedly mounted-toasupport memher for rotatably supporting-.- the anode whereby the rotatable anode is completely enclosed by the shield, the external surfaceot which is smooth to prevent cold and. secondary emission. therefrom, said disc-shaped portion having; an aper-- ture therein tov permitelectronn emitted by said cathode electrode to impinge onsaid anode electrode. and. said cylindrical portion having an aperture therein aligned. with. an apertured pore ti-on in said metallic wall. portiomtopermit the passage of x rays.

4. An Y-ray tube comprising an evacuated tubular envelope having a metallic wall portion, a rotatable anode electrode within said envelope, a cathode electrode within said envelope, and a shield member comprising a cylindrical portion surrounding the anode and a disc-shaped portion closing the cylindrical portion interposed between the anode and cathode electrodes, said cylindrical portion being fixedly mounted to a support member for rotatably supporting the anode whereby the rotatable anode is completely enclosed within the shield member, the external surface of which is smooth to prevent secondary and cold emission therefrom, said disc-shaped portion having an aperture therein eccentricali disposed with respect to the axis of rotation of said anode to permit electrons emitted by said cathode electrode to impinge on said anode electrode and said cylindrical portion having an aperture therein aligned with an apertured portion in said metallic wall portion to permit the passage of X-rays.

5. An X-ray tube comprising an evacuated tubular envelope having a metallic wall portion, a rotatable anode electrode within said envelope,

magnet means for imparting rotation to said anode electrode, a cathode electrode within said envelope, a shield member comprising a cylindrical portion surrounding the anode and a disc-shaped portion closing the cylindrical portion interposed between the anode and cathode electrodes, said cylindrical portion being fixedly mounted to a support for rotatably supporting the anode electrode whereby the rotatable anode is completely enclosed within the shield member, said discshaped portion having an aperture therein eccentrically disposed with respect to the axis of r0- tation of said anode to permit electrons emitted by said cathode electrode to impinge on said anode electrode, said cylindrical portion having an aperture therein aligned with an apertured portion in said metallic wall portion to .permit the passage of X-rays and apertures therein to permit the passage of magnetic flux to cause said anode to rotate.

6. An X-ray tube comprising an evacuated tubular envelope having a metallic wall portion, a rotatable anode electrode within said envelope, a support member for rotatably supporting said anode, magnet means for impartin rotation to said anode electrode, a cathode electrode within said envelope, and a shield member comprising a cylindrical member surrounding the anode and a disc-shaped member closing the cylindrical member interposed between the anode and cathode electrodes, the cylindrical portion being fixedly mounted to said supporting member for said anode and in heat transfer relationship thereto for conducting heat away from said anode whereby said rotatable anode is completely enclosed within the shield member, said disc-shaped portion having an aperture therein eccentrically lobated with respect to the axis of rotation of said anode to permit electrons emitted by said cathode electrode to impinge on said anode electrode, said cylindrical portion having an aperture therein aligned with an apertured portion in said metallic wall portion to permit the passage of X-rays and apertures therein to permit the passage of magnetic flux to cause said anode to rotate.

JACOB HARMANNUS VAN DER TUUK. JOHANNES JAN SMIT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transient Temperatures in the Anode of an X-Ray Tube, reprinted from Journal of Applied Physics, vol. 13, N0. 6, 384-389, June 1942; copyright 1942 by the American Institute of Physics; F. R. Abbott, Vacuum Tube Engineering Department, General Electric X-Ray Corporation, Chii cago, Illinois. 

